Commercial vehicles have for many years been fitted with servo braking systems which apply brake fluid, which may be a hydraulic fluid or air, to brake actuator cylinders in response to control inputs from the vehicle driver. Pressure of the driver's foot on a brake pedal controls the flow of brake fluid to operate the brake actuator cylinders and apply brake shoes or pads to the vehicle wheel hubs or brake discs, respectively. The fluid is provided to the actuator cylinder from a high-pressure source via a servo device. By this means, the force applied by the driver to the pedal is amplified to the levels necessary to arrest the movement of a heavy vehicle.
In commercial vehicles, it is desirable for the vehicle to be able to carry a large payload in proportion to the unladen weight of the vehicle, and thus there is great variation between the unladen and fully laden weights of such vehicles. When the vehicle is unladen, deceleration can be achieved satisfactorily with relatively low fluid pressures in the brake actuator cylinders. As the total weight of the vehicle increases, braking requires higher fluid pressures in the brake actuator cylinders in order to produce the same deceleration rate. It is also necessary to provide a braking system which provides predictability to the driver, by giving the same or similar deceleration rates for similar pedal pressures at any loading state of the vehicle. This is achieved by providing a braking system which, for the same pedal pressure applied by the driver, applies less fluid pressure to the brake actuator cylinders when the vehicle is unladen than when it is heavily loaded.
To effect such a control of the vehicle braking system, it is conventional to provide a throttling valve, known as the “load sensing valve” in the fluid circuit supplying the brake actuator cylinders. The load sensing valve comprises a valve body and an operating arm, the valve body having a passage provided with a variable throttling element whose throttling effect is varied by moving the operating arm. The valve body is conventionally fixed relative to the vehicle body and the operating arm is attached to an axle of the vehicle on spring-suspended vehicles. This arrangement may however be reversed. As more load is placed on the vehicle, the suspension springs are compressed, and the distance between the points to which the load sensing valve is attached varies as the axle moves nearer to the vehicle body. The compression of the suspension springs progressively reduces the “ride height” of the vehicle as it is more heavily loaded, and acts as an indicator of the weight of the vehicle. An individual correlation will therefore exist between the loading state and the ride height of the vehicle, depending on the characteristics of the suspension springs. The driver of the vehicle will become accustomed to the braking performance of the vehicle at various loading states.
The operation of the load sensing valve is to provide a strong throttling action to reduce the flow of brake fluid to the brake actuator cylinders when the vehicle is lightly loaded, and when the vehicle is heavily loaded to provide little or no throttling action and allow brake fluid to flow unimpeded to the brake actuator cylinders when the driver applies pedal pressure. The actual braking effect generated by the brake actuator cylinders thus increases as the vehicle is more heavily loaded. Each vehicle has a design relationship between the gross weight and the braking amplification factor, calling for a predetermined variation of the degree of throttling provided by the load sensing valve over the range of vehicle weight from unladen to maximum gross weight. Typically, the load sensing valve will reduce the brake fluid pressure by some 1500 psi when the vehicle is unladen, and will allow free flow when the vehicle is at its maximum gross weight.
In recent years the use of “air suspension” has become widespread in heavy goods vehicles. However, in applying this technology to light goods and passenger vehicles, vans or the like a significant difficulty has emerged as regards the variation of braking performance with vehicle weight.
In vehicles using air suspension, the vehicle is supported on its axles not by springs but by suspension units or “bags” filled with air under pressure. The “bags” may be flexible structures of toroidal or other form, or may be telescoping structures having sliding or rolling diaphragm seals. The “ride height” of the vehicle is controlled by varying the pressure within the bags, and thus is no longer dictated by the gross vehicle weight. The bags may also be inflated or deflated to raise or lower the vehicle body in relation to the ground, this feature being of great assistance in loading the vehicle, since by lowering the vehicle body the height to which cargo need be lifted to enter the vehicle's loadspace is reduced.
It has been found that the handling and “driveability” of the vehicle is improved by adopting a control system for the bag pressure which adjusts the ride height to a maintain it at a constant level slightly below the unladen position. Such a control provides for a predetermined amount of suspension travel at all loading states, to maintain the ground clearance of the vehicle at a required distance. Ride height control may be achieved by admitting air into or venting air from the bags in response to a measurement of ride height. Maintaining a constant ride height however means that the ride height cannot be used as an indicator of the vehicle's gross weight in a control arrangement for the braking system.